The effects of intravenous infusion of vasopressin and oxytocin at different dose rates on systemic arterial pressure, heart rate, hepatic artery resistance and vascular resistance in the superior mesenteric bed will be studied in anesthetized dogs. The phenomenon of tachyphylaxis to repeated administration of posterior pituitary hormones will be investigated. The interaction of these hormones with autonomic mechanisms will also be explored by studying the effect of adrenergic blocking agents on their vascular action and by studying the effect of adrenergic blocking agents on their vascular action and by studying the effect of vasopressin on the pressor response to norepinephrine. The combined action of posterior putuitary hormones and estrogens or progesterone at peripheral vasomotor receptor sites will also be studied.